This invention relates to a rear projection display device so adapted as to receive an image projected from an image projector (an optical engine) on a rear surface of a screen and to project image light toward an observer positioned in front of the screen.
FIG. 10 is a schematic view illustrating a structure of a conventional rear projection display device. The rear projection display device comprises an image projector (an optical engine) 102, a projection lens 103, a reflective mirror 104 and a transmission-type diffusion screen 105 in a case 101. Image light projected from the image projector 102 is projected and magnified through the projection lens 103, reflected on the reflective mirror 104 and received on a rear surface of the transmission-type diffusion screen 105. An observer can watch the image in front of the diffusion screen 105. This type of the rear projection display device has an advantage in that upsizing of a screen is easily achieved in comparison with display devices using a cathode-ray tube.
In order to realize upsizing of the screen, the above-mentioned rear projection display device requires either using the projection lens 103 with a high magnification or extending an optical path between the image projector 102 and the screen 105. The projection lens has a limit to magnification under present circumstances. When providing a projection lens of a high magnification, the case 101 also increases its depth, leading to the difficulties in slimming. When extending the optical path between the image projector 102 and the screen 105, the case 101 also increases its depth, leading the difficulties in slimming.
On the other hand, JP 111458/1998 A (Int. Cl G02B 17/00) discloses another conventional rear projection display device including an image forming system comprised of three reflective mirrors having curved surfaces.
The image forming system comprises a first reflective mirror whose reflective surface is spherical and concave in shape, a second reflective mirror whose reflective surface is aspherical and convex in shape, and a third reflective mirror whose reflective surface is aspherical and convex in shape. Image light optically modulated in a liquid crystal panel is reflected on the first to third reflective mirrors in sequence and irradiated on the screen.
The optical path is folded up by the first, second and third reflective mirrors, therefore the depth of the case can be reduced.
To make the case slimmer, however, image light should be emitted at an angle with respect to the screen, specifically; a main light beam of the image light should be emitted at an angle with respect to a normal to the screen. Under the circumstance, it is difficult to achieve power arrangement of reflective mirrors enough to correct aberrations such as astigmatism and coma only by aspherical components of the second and third reflective mirrors.
The mirror unit for forming and projecting image comprised of the first to third reflective mirrors is an optical part having a same function as that of a general projection lens. Considering efficiency of adjusting operations in assembling processes of the device, positioning accuracy of each mirror, and dust-resistance effect, it is preferable to shorten the distance between mirrors and integrally hold the mirrors in a holding body.
Each of the first to third mirrors, however, is placed in close to a front or rear side of the case. When each of the reflective mirrors is held by an integral mechanism, a holding system becomes big. Therefore it is difficult to hold the mirrors and place a dust-resistance mechanism by using the integral mechanism
In a rear projection display device comprising an image displaying liquid crystal panel of about 1 inch diagonally arranged in the optical engine, and magnifying and projecting an image onto a 50 inch diagonal screen, for example, the spacing between mirrors requires 20-30 cm with consideration given to the power arrangement of each mirror and image forming performance such as MTF. Therefore, it is difficult to integrally hold every mirror. Also some problems are found in assembling efficiency of the rear projection display device, positioning accuracy of the mirrors, and dust-resistance effect.
Generally, one of the biggest problems of a reflective-type image forming optical system incorporating reflective mirrors is interference between reflected light flux and members in the device. In the display device using three reflective mirrors, the light flux, which travels from the first reflective mirror to the second reflective mirror, passes in proximity to the upper part of the optical engine, which may cause interference between the light flux and the holding system for the image forming members. That places constraint on a design of the holding system and the mirror optical system.
This invention was made to solve the above-mentioned problems and has an objective to reduce the depth of the display device.
Also, this invention has an objective to provide a rear projection display device capable of sending image light to a magnifying and projecting system without being cut off by the optical engine or the like.
Further, this invention has an objective to reduce the depth of the rear projection display device by projecting image light at an angle with respect to the screen without the first reflective mirror in an aspherical shape which is complicated to manufacture.
This invention comprises an optical engine including an image forming unit for forming image information by spaciously modulating intensity of a light flux from a lightning unit including a light source, a first reflective mirror in a concave shape arranged opposite to the optical engine to receive the light flux from the optical engine, a second reflective mirror in a convex shape arranged opposite to the first reflective mirror, a third reflective mirror in a convex shape arranged opposite to the second reflective mirror, a fourth reflective mirror in a convex shape arranged opposite to the third reflective mirror, a plane reflective mirror arranged opposite to the fourth reflective mirror, and a screen on which an image from the plane reflective mirror is displayed. The second reflective mirror is in a position in proximity to an upper part of the optical engine and closer to the first reflective mirror than the optical engine is. Also the first, second, third reflective mirrors are held in an integral holding system.
The described invention adopts four curved mirrors in all including a new curved reflective mirror arranged in proximity to the upper part of the optical engine, thereby preventing interference of the light flux reflected between reflective mirrors and the optical engine. Also the invention can improve assembling efficiency of the rear projection display device and positioning accuracy of the reflective mirrors and have dust-resistance effect by integrally holding the new reflective mirror arranged in proximity to the optical engine and other reflective mirrors around it in the holding system.
The rear projection display device according to the invention comprises an optical engine which optically modulates light emitted from a light source on the basis of image information and emits the modulated light, and projecting means for magnifying and projecting the emitted light from the optical engine through a plurality of reflective mirrors to a rear surface of the screen at an angle. In the rear projection display device which an image is observed from a front surface of the screen, the plurality of reflective mirrors includes at least one or more than one curved reflective mirror. An auxiliary lens for correcting an aberration of the curved reflective mirror to which light emitted from the optical engine is first irradiated is arranged on the way to the curved reflective mirror and closer to the optical engine.
According to such a structure, an image light emitted from the optical unit is magnified and corrected for aberration by the projection means including curved reflective mirror and the auxiliary lens, and emitted at an angle to the screen. Then, power of the curved reflective mirror, to which the light emitted from the optical engine is first irradiated and which requires the highest accuracy, is corrected by the auxiliary lens. Although the image light emitted from the optical unit is magnified in sequence as the image light travels closer to the screen, the auxiliary lens can be small because it is arranged between the optical unit and the curved reflective mirror where the light flux passing through is the smallest.
The projecting means in this invention comprises a first curved reflective mirror having a concave surface opposite to the light flux from the optical engine, a second curved reflective mirror having a convex surface opposite to the light flux from the first curved reflective mirror, a third curved reflective mirror having a convex surface opposite to the light flux from the second curved reflective mirror, a fourth reflective mirror for introducing the light flux from the third curved reflective mirror to a back surface of the screen and an auxiliary lens arranged on an optical path from the optical engine to the first curved reflective mirror.
According to such a structure, an image light emitted from the optical unit passes through the auxiliary lens, is reflected on the first curved reflective mirror, the second curved reflective mirror and the third curved reflective mirror in this order and is irradiated on the screen.
Also the invention is characterized by comprising a first curved reflective mirror having a spherical reflective surface and an aspherical auxiliary lens.
According to such a structure, the spherical reflective surface of the first curved reflective mirror is formed with high accuracy as well as the auxiliary lens which compensates aspherical components. The reflective surface and the auxiliary lens can be accurately formed by a conventional polishing method, because the reflective surface of the curved reflective mirror is spherical in shape and the auxiliary lens is relatively small.
The invention is characterized by that the second and third curved reflective mirrors have aspherical reflective surfaces.
The second and third curved reflective mirror can be formed by a conventional polishing method, because they are big in comparison with the first curved reflective mirror and require moderate accuracy.